The present invention relates to force-sensitive transducers, and more particularly, to force-sensitive transducers which function as hand-operated controllers for a variety of applications such as machine tools, computer terminal cursor controllers, remote vehicle controls and other devices.
Ergonomic studies have demonstrated that force-responsive controllers are preferable to displacement-responsive controllers for a wide range of applications. Force-responsive controllers provide the operator with enhanced control of the transducer apparatus between the operator and the machine being controlled, by responding to finger pressure rather than displacement. In contrast, displacement-responsive joystick controllers give less positive control, and frequently suffer from excessive backlash as well as lack of tactile feedback in the area around the spring-loaded neutral position.
Force transducers employing capacitive sensing are known in the art. The following U.S. patents are illustrative of recent developments in force-sensitive transducers.
Mehr, U.S. Pat. No. 3,270,260 PA1 Lee et al., U.S. Pat. No. 3,859,575 PA1 Bell, U.S. Pat. No. 4,295,376 PA1 Sharp et al., U.S. Pat. No. 4,493,219
U.S. Pat. No. 3,270,260 to Mehr discloses a stick-operated controller utilizing a semi-rigid conductive diaphragm responsive to forces applied to the stick. The diaphragm, together with pickup plates separated from the parallel to the diaphragm, defines capacitors having capacitance varying as a function of diaphragm-to-pickup plate spacing. The Mehr device accordingly measures force applied to the stick by measuring the out-of-plane deflection of the diaphram.
U.S. Pat. No. 3,859,575 to Lee discloses a capacitive transducer having a flexible diaphragm deflectable in response to forces applied in a direction normal to the plane of the diaphragm.
Pat. No. U.S. 4,295,376 to Bell discloses a capacitive transducer having an electrode-carrying diaphragm responsive to forces which act in a direction normal to the diaphragm's plane of repose, causing the diaphragm to assume a dome-like shape. The diaphragm is fixed by a peripheral mount. An intermediate spacer element, disposed concentrically within the peripheral mount, provides support for a rigid reference plate which carries a pair of reference electrodes. Because the spacer element is affixed to a flexible portion of the diaphragm, special construction of the spacer element is required to prevent constraint of the diaphragm by the spacer element.
In the Bell device deflection is measured in the two radially symmetric diaphragm regions delimited by the spacer element and the peripheral mount. The pair of reference electrodes separated from the diaphragm-mounted electrodes define, together with the diaphragm-mounted electrodes, capacitors having capacitance variable with diaphragm-to-reference gaps. The Bell device accordingly measures normal force applied to the diaphragm, by measuring out-of-plane deflection of the diaphragm,
U.S. Pat. No. 4,493,219 to Sharp discloses a energy conversion and transmission device utilizing a joystick connected to a plurality of elastic pads, in connection with capacitive transducing elements.
Prior art force transducers like that disclosed in Mehr suffer from poor sensitivity to user input. This poor senstivity is due to the prior art approach of measuring the out-of-plane deflection of the diaphragm. The out-of-plane deflection of the diaphragm is small, and the area of maximum deflection of the diaphragm is limited. In the Mehr device, for example, deflection of the diaphragm is greatest near the central region of the diaphragm, where the area available for the measuring capacitors is small.
Prior art capacitive transducers also suffer from the disadvantage of high impedance for a given gap dimension. Smaller gap sizes have been used in an attempt to reduce impedance, and especially, to increase sensitivity, but smaller gap sizes result in critical alignment requirements. Additionally, proper alignment of the capacitive elements in prior art transducers is difficult to achieve, in view of such transducers' complex construction, small gap dimensions and parallelism requirements.
There accordingly exists a need for a force-sensitive transducer having enhanced sensitivity to force input, reduced impedance, minimal alignment problems, and simplified construction, which can be constructed at low cost.